1. Field of the Invention
This invention relates to exposure control devices for cameras and more particularly to exposure control devices for cameras having half-open type shutters whose blades, upon opening and closing, also serve as diaphragm blades to control the exposure.
2. Description of the Prior Art
In cameras having diaphragm-cum-shutter blades or so-called half-open type shutters, where exposures dependent upon the opening and closing of the shutter blades are accurately controlled with light metering information, a photosensitive element is placed in the optical path and a diaphragm in the form of an auxiliary aperture is arranged to open and close when the main diaphragm aperture for exposure of film opened and closed, so that the amount of light to the photosensitive element can be varied in proportion to the amount of light to the film. However, because the auxiliary diaphragm aperture is closed before initiation of the opening of the shutter blades, it becomes impossible to presense the level of brightness of an object to be photographed just before actuation of a shutter release to detect an inadequate brightness level and produce a warning signal.
To solve this problem, it is necessary that the auxiliary diaphragm aperture be opened before the shutter is release. Upon initiation of a shutter blade opening operation, it is necessary to change the auxiliary aperture from the pre-opened state to a totally closed state and to have an exposure control circuit control the closing operation of the shutter by starting to count light-metering information with a photosensitive element. However, a mechanical count start switch cooperating with the shutter blades is disadvantageous to the reliability of the opening operation of the shutter blades because it shocks the shutter blades when actuated. It also adversely affects the accuracy of exposure control because the timing of actuation becomes unstable. Therefore, it has been the conventional practice, for example, to utilize the change in the output of the photosensitive element of the light meter resulting from the shutter operation's changing the opening area of the auxiliary diaphragm aperture from the pre-opened state to the totally closed state in actuating the exposure control circuit to initiate a counting operation, as proposed in U.S. Pat. No. 4,258,990 (issued Mar. 31, 1981).
However, such devices, because they determine when the counting operation starts on the basis of the output of the photosensitive element in the light meter, are unsuitable for night photography. This is so because the object brightness is so low that the output of the photosensitive element with the pre-opened auxiliary diaphragm aperture is almost equal to that with the auxiliary diaphragm aperture totally closed. Hence, the possibility of being unable to detect the difference between the two outputs exists. For this reason, one may encounter quite a few photographic situations where such a device is unreliable for starting a counting operation.
It may also be possible to employ a timer circuit responsive to initiation of a shutter operation for starting the count and for producing an actuating signal for the exposure control circuit at the time necessary for moving the auxiliary diaphragm aperture from the pre-opened position to the totally closed position. However, such a method requires always opening the shutter blades at a constant speed and also strictly adjusting the timing of actuation of each mechanical part. These requirements are difficult to fulfill.